Metal-ceramic composites for tribological uses and defined sliding/friction pairs based on said materials

ABSTRACT

The friction heat generated between both sliding pairs and friction pairs has to be swiftly dissipated in older to maintain a lubricant film or ensure constant coefficients of friction. The invention consequently relates to metal-ceramic composites for friction/sliding uses, which are characterized by basic compositions containing 30 to 75 percent by volume of one or several metallic phases, preferably aluminum and the alloys thereof, and 25 to 70 percent by volume of one or several non-metallic inorganic component/s as ceramic materials, preferably silicon carbide, aluminum oxide, titanium oxide, and silicates.

The invention relates to metal-ceramic composites for slide/friction applications in which, to guarantee or improve performance, the materials/material combinations used have a high thermal conductivity and/or produce low frictional heat and/or tend to have low static friction, and to defined slide/friction pairings based on these composites.

For both slide pairings and friction pairings, the frictional heat produced must be dissipated rapidly away from the friction/slide region. This is necessary in order to maintain a lubricating film or guarantee constant coefficients of friction. The slide pairings used according to the state of the art, especially in the mixed friction region or for dry running, are based on silicon carbide against carbon, as described e.g. in W. Tietze, Handbuch Dichtungspraxis, 2nd edition, Vulkan-Verlag, 2000.

The low thermal conductivity of carbon, e.g. 8 to 17 W/mK according to the material characteristics from Schunk Kohlenstofftechnik GmbH, Technologien in Kohlenstoff, Geschaftsbereich 1, Lager-und Dichtungs-technik, Werkstoffkennwerte, Standardwerkstoffe, Schunk, 30.14 (1997), can lead to a noticeable temperature increase in the gap. Such a temperature increase gives rise to thermal stressing of the binder system and the impregnation, which can cause changes in the materials, these in turn leading to unfavourable slide conditions. The tribological properties can become impaired despite the very good thermal conductivity of silicon carbide of e.g. 80 to 130 W/mK.

Temperature increases in the slide partners affect the fluid in the gap and modify the friction/slide conditions. Solubilizers can crystallize out due to the temperature change, which, after a period of rest, increases the breakaway/starting torque and, in the worst case, leads to seizure of the slide pairing.

This can occur both with rotating sealing units and with those undergoing translational movement.

Low mechanical values such as the tensile strength, flexural strength and hardness of a slide partner, for example carbon with values of 30 to 80 MPa, additionally restrict the range of uses of the above-mentioned sealing units. Fillers, particularly impregnations, may be partially attacked. Chemically aggressive media cause the impregnations to swell and thereby change the tribological conditions. This is a further possible cause of a temperature increase in the gap.

Temperature and pressure change the geometry and hence the original setting of a slide pairing, which as a rule impairs performance.

The object of the present invention is to provide a favourable friction/slide system which meets the following requirements: constant friction/slide properties, high thermal conductivity, dimensional stability due to high modulus of elasticity, and high strength.

This object is achieved by selecting specific materials and material pairings.

Materials according to the invention that have the requisite properties include metal-ceramic composites, or MCCs, consisting of one or more metallic phases in a proportion of 30 to 75 vol. % and one or more non-metallic inorganic components in a proportion of 25 to 70 vol. %. Preferred metallic phases are aluminium and its alloys. The non-metallic ceramic components are ceramic materials, preferably silicon carbides, aluminium oxides, titanium oxides and silicates.

One of the preferred MCCs, based on Al₂O₃ and Al, has a composition of 40 to 60 vol. % of Al₂O₃ and 60 to 40 vol. % of Al with a thermal conductivity of >50 W/mK, a flexural strength of e.g. 300 MPa and a modulus of elasticity of e.g. 160 GPa.

Another preferred MCC, based on SiC and Al, has a composition of 60 to 80 vol. % of SiC and 40 to 20 vol. % of Al with a thermal conductivity of e.g. 180 W/mK, a flexural strength of e.g. 300 MPa and a modulus of elasticity of e.g. 200 GPa.

Metal-ceramic composites with a metal content of more than 50 vol. % are called metal-matrix composites (MMCs). If the ceramic content is more than 50 vol. %, the materials are called ceramic-matrix composites (CMCs).

Surface finishes with Ra values below 1 μm are achieved on the machined operating surfaces. However, these can be varied by means of appropriate hard machining processes and thus optimized according to the friction partner.

The heat build-up in friction/slide applications is reduced by the choice of materials for the MCCs and by the surface finish. In addition, the heat generated in the sealing gap is rapidly dissipated to the surroundings by virtue of the high thermal conductivity of the materials. The sealing gap temperature is thereby lowered and cracking, efflorescence and deposition in the sealing gap are substantially reduced, resulting in more favourable and more constant coefficients of friction and abrasion factors.

Moreover, the improved dimensional stability reduces the possibility of edge running. This lowers the temperature peaks and produces more stable fluid films in the sealing gap, which in turn reduces the frictional activity and heat build-up.

In so-called hard/soft pairings, replacement of the softer partner, for example carbon or plastic, by a ceramic material or a composite results in improved mechanical properties for the whole system and thereby expands the possible uses. Furthermore, the tribological conditions are improved by avoiding friction partners which tend to swell and/or behave in a critical manner towards chemical attack. The pairing can thus be designed with narrower tolerance limits than the materials used hitherto.

In so-called hard/hard pairings, the dry running or the emergency running properties of the tribological pairings are improved by using MCCs.

Reducing the abrasion lengthens the life of the friction pairings and extends the service intervals.

Using the materials and material pairings according to the invention brings decisive improvements in the industrial sector, especially in the automotive industry and consumer goods industry.

In general, by specifically designing the material with a chosen combination of ceramic and metal in the penetration structure, it is possible to adapt the tribological properties of the pairings for the applications listed below by way of example.

Axial face seals in cooling water pumps, especially MCC/carbon, MCC/Al₂O₃, MCC/SSiC, MCC/MCC, MCC/HM (hard metal), MCC/ZTA (Al₂O₃+ZrO₂)

Axial face seals in dishwashing machines, especially MCC/carbon, MCC/Al₂O₃, MCC/SSiC, MCC/MCC, MCC/HM, MCC/plastic, optionally fibre-reinforced.

Axial face seals in direct fuel injection pumps, especially MCC/carbon, MCC/SSiC, MCC/Al₂O₃, MCC/MCC, MCC/HM, MCC/ZTA.

Axial face seals in CO₂ compressors, especially MCC/SSiC, MCC/Al₂O₃, MCC/MCC, MCC/HM, MCC/ZTA.

Defined slide/friction pairings between braking, bearing, sealing or drive units, for example in lifts, escalators, cranes, dry couplings, pumps and compressors, pistons and cylinders, swash plates, radial bearings or axial bearings, bearings of grinding cylinders, and slide partners for rotary shaft seals.

Choosing the materials of the friction partners specifically in each case opens up new fields of application, especially in lubricant-free areas of use, e.g. in the pharmaceutical and cosmetic industry or in food technology.

The materials according to the invention are recommended as friction partners particularly in cases where, after periods of rest, the problem of seizure or deposition in the sealing gap occurs and increases the breakaway torque, said materials being used e.g. as gaskets for espresso machine fittings, sanitary ware fittings and industrial fittings or for shut-off valves.

Other possible applications are side plates in fuel pumps or power-assisted steering pumps. Apart from abrasion resistance, these applications require in particular a high dimensional stability, which is guaranteed by virtue of the low contraction of the components. This increases the efficiency of the pumps, enabling smaller, more compact and lighter designs to be used.

One possible use is as rollers and/or bearing units, e.g. in combustion engines, compressors or exhaust valves.

Use in highly stressed valve drives, such as those present in diesel engines, represents an alternative to today's expensive solutions.

As a rule, slide units are specially constructed according to the client's drawings and specifications. Typical dimensions for mass-produced sliding rings are as follows: external diameter: 18 to 28 mm, internal diameter: 8 to 20 mm, and height: 2 to 5 mm. 

1-18. (canceled)
 19. A metal-ceramic composite for friction/slide application, comprising a base composition comprising at least one metallic phase in a proportion of 30 to 75 vol. % and at least one non-metallic inorganic component in a proportion of 25 to 70 vol. % as a ceramic material the composite having a thermal conductivity greater than 50 W/mK, a flexural strength of about 300 MPa and a modulas of elasticity of at least 160 GPa.
 20. A metal-ceramic composite according to claim 19, wherein the composition contains 40 to 60 vol. % of Al₂O₃ and 60 to 40 vol. % of Al.
 21. A metal-ceramic composite according to claim 20, wherein the thermal conductivity is greater than 50 W/mK, the flexural strength is about 300 MPa and the modulus of elasticity is about 160 GPa.
 22. A metal-ceramic composite according to claim 19, wherein the composition contains 60 to 80 vol. % of SiC and 40 to 20 vol. % of Al.
 23. A metal-ceramic composite according to claim 22, wherein the thermal conductivity is at least 180 W/mK, the flexural strength is about 300 MPa and the modulus of elasticity is about 200 GPa.
 24. A metal-ceramic composite according to claim 19, wherein the surfaces in contact with a friction/slide partner have an Ra value below 1 μm.
 25. A sliding ring comprising a metal-ceramic composite of claim
 19. 26. A slide/friction pairing comprising a metal-ceramic composites according to claim 1, wherein the pairings comprise one partner consisting of a metal-ceramic composite (MCC) and one partner consisting of MCC, carbon, Al₂O₃, SSiC, hard metal (HM), ZTA (Al₂O₃ and ZrO₂) or plastic, optionally fibre-reinforced.
 27. A slide/friction pairing according to claim 26, selected from the group consisting of MCC/carbon, MCC/Al₂O₃, MCC/SSiC, MCC/MCC, MCC/HM and MCC/ZTA (Al₂O₃+ZrO₂).
 28. A slide/friction pairing according to claim 26, selected from the group consisting of MCC/carbon, MCC/Al₂O₃, MCC/SSiC, MCC/MCC, MCC/HM and MCC/plastic, optionally fibre-reinforced.
 29. A slide/friction pairing according to claim 26, selected from the group consisting of MCC/carbon, MCC/SSiC, MCC/Al₂O₃, MCC/MCC, MCC/HM and MCC/ZTA.
 30. A slide/friction pairing according to claim 26, selected from the group consisting of MCC/SSiC, MCC/Al₂O₃, MCC/MCC, MCC/HM and MCC/ZTA.
 31. The metal-ceramic composite of claim 19, wherein said at least one metallic phase comprises aluminum or an aluminum alloy.
 32. The metal-ceramic composite of claim 19, wherein said ceramic material is selected from the group consisting of silicon carnie, aluminum oxide, titanium oxide and a silicate. 